The present invention relates to an automatic transmission vacuum modulator assembly having a vacuum modulator with a spring and a diaphragm moveably linked to a modulator sliding or spool valve, wherein the assembly adjusts the hydraulic pressure within an automatic transmission.
Many vehicles with diesel engines are equipped with a Ford.RTM. C-6 automatic transmission, a fully automatic transmission with three forward speeds and one reverse speed. The transmission consists of a welded torque converter assembly, a two unit planetary gear train, and a hydraulic system to control gear selection and automatic shifts. This Ford.RTM. C-6 transmission is installed in most emergency vehicles, such as ambulances and fire engines, and in many mid-sized trucks.
The transmission is controlled automatically by a hydraulic system in which hydraulic pressure is supplied by an engine-driven transmission oil pump. Main line pressure is controlled by a pressure regulator valve train and by the vacuum modulator. The pressure regulator valve train controls line pressure automatically, in response to a pressure signal from a modulator valve, in such a way that the torque requirements of the transmission clutches are met and proper shift spacing is obtained at most throttle openings.
To control line pressure properly, a modulator pressure is used which varies in the same manner as torque input to the transmission. In a gas engine, the modulator pressure is regulated by engine vacuum which is an indicator of engine torque and carburetor opening. The vacuum modulator is connected at one end to engine vacuum and at the other end to the modulator valve. By contrast, in a diesel engine, vacuum is provided by an auxiliary vacuum pump, and is regulated by means of a vacuum pressure regulator valve located on the injector pump. The vacuum modulator in a diesel engine is thus connected at one end to the vacuum regulator, which is in turn connected to the vacuum pump.
The vacuum modulator mainly comprises a housing, a heavy-gauge spring and a diaphragm. Based upon the vacuum asserted on the modulator, the spring applies a force that acts upon the modulator valve via a connecting pin. When the vacuum is low or zero, the spring force acts on the modulator valve to increase modulator pressure. By contrast, high vacuum pressure acts on the diaphragm to offset spring force and decrease modulator pressure. Thus, low vacuum creates a high pressure condition and high vacuum creates a low pressure condition.
When the modulator vacuum valve is adjusted to provide proper timing and shift feel at closed and part throttle positions, full throttle up-shifts are not normally possible. That is, when there is a low vacuum acting on the vacuum modulator and corresponding high modulator pressure, the valve is in a disadvantageous high pressure or closed position. Thus, the vehicle is in a full throttle ("foot on the floor") position and up-shifts are not possible without decreasing the throttle opening from full throttle ("letting up on the pedal") for a shift to occur. With the Ford.RTM. diesel transmission engines especially, rapid acceleration from a standstill or rapid acceleration up a steep hill, when vacuum is low, is not possible without decreasing the throttle from a full throttle position.
Especially for emergency vehicles with a diesel engine and a Ford.RTM. C-6 automatic transmission, there is a need for a modulator valve that will permit more rapid, yet smooth, gear shifts. Transmission specialists have attempted to solve this problem by reducing the modulator spring pressure acting on the modulator valve when there is a low vacuum/high pressure condition in the valve. The modulator spring force/tension may be adjusted by turning a setting screw. This adjustment does permit upshifts when the valve is in the low vacuum/high pressure position. However, when the modulator spring is adjusted to a position that allows for full throttle upshifts during a high pressure position of the valve, closed and part throttle upshifts are usually too early, too mild, or stacked too close together. Such adjustments to the modulator spring thus adversely affect the modulator at low pressure positions.
Vacuum modulator assemblies of a different design to permit upshifts under special conditions are known. One such example in some General Motors.RTM. transmissions consists of a housing, an evacuated metal bellows, a diaphragm and two springs. When installed, the bellows and its external spring apply a force that acts on the modulator valve to increase modulator pressure. Engine vacuum and the other spring within the bellows act in the opposite direction to decrease modulator pressure. The effective area of the diaphragm is somewhat larger than the effective area of the bellows to use atmospheric pressure to compensate for engine power loss at certain conditions, such as at high altitudes. The addition of the bellows and the second spring within the bellows give the modulator assembly in these engines an operating range that spans a greater range of modulator pressures. However, these altitude compensating vacuum modulator assemblies correct a different problem and still would not permit a workable line pressure to allow shifting to occur when there is a low vacuum acting on the modulator.